Cell therapy using bone marrow (BM)-derived stem or progenitor cells has emerged as a promising modality for repairing or regenerating ischemic cardiovascular diseases. Many experimental studies have demonstrated that BM-derived cells such as endothelial progenitor cells (EPCs), mesenchymal stem cells (MSCs) or similar multipotent stem cells are effective to repair or regenerate ischemic cardiovascular diseases. In the meantime, there has been an ongoing controversy regarding the (trans)differentiation potential of BM-derived stem or progenitor cells. It is now more widely accepted that the therapeutic effects of BM-derived EPCs are mediated mainly through non-transdifferentiation effects such as paracrine or humoral effects. Furthermore, the culture-identification of newer types of EPCs in human has rendered this field more complicated. However, no one has yet reported a specific marker to prospectively isolate EPCs derived from BM. Over the last few years, my lab has been investigating two most important topics in EPC biology: transdifferentiation and therapeutic effects in cardiovascular diseases. During this research, we have found that approximately 25-30% of BM mononuclear cells express the CD31 epitope, and these BM-derived CD31-positive (BM-CD31+) cells have high angiogenic activity and include vasculogenic (having the potential to differentiate into endothelial cells) cell population(s). We have also found that these CD31+ cells are highly effective for treating limb ischemia. Accordingly, in this proposal, we propose to further identify novel EPCs by sub- fractioning CD31+ cells using a panel of candidate markers, which have genuine (trans)differentiation potential and/or angiogenic/paracrine activities. In Specific aim 1, we will explore identification of novel EPCs contained in CD31+ fractions which possess vasculogenic potential. In Specific Aim 2, we will the angiogenic or paracrine properties of these CD31 sub-populations. We anticipate that the results of the experiments outlined in this proposal will yield new insight into the identity and therapeutic potency of novel EPCs for repairing ischemic cardiovascular diseases. PUBLIC HEALTH RELEVANCE: Endothelial progenitor cells (EPCs) from bone marrow have shown the potential to repair or regenerate ischemic cardiovascular tissues;however, there still remain concerns regarding the identity and underlying therapeutic mechanisms. We recently identified a novel population of bone marrow-derived CD31 positive cells which have angiogenic and genuine vasculogenic properties. In this proposal, we propose to identify genuine EPCs which have vasculogenic and/or angiogenic properties by narrow downing the CD31 subfractions.